Marchllyn slate quarry is located immediately below Marchllyn Bach Reservoir, to the north-east of Dinorwig. It represents a late attempt by the Dinorwig company to re-develop on a new site in order to win slate more cheaply than at Dinorwig quarry itself. The latter had become expensive to work because of its dispersion and to the indiscriminate early rubbish dumping on reserves of good slate. At the new site a major investment made in the 1950s included an electric mill, but it occurred at a time of low prices and shortage of capital so the enterprise failed and it closed in 1969. It was portrayed in operation of the Ordnance Survey six-inch map of 1953 where it is referred to as 'Dinorwic Quarry'.

Additional sources:
RCAHMW air photos: AP94-CS 1047 & AP945133/50.
A.J.Richards, A Gazeteer of the Welsh Slate Industry (1991), p.40.

RCAHMW, 16 February 2015